Method for hot dip coating of metal strip



Dec. 11, 1951 c. M NABB ETAL METHOD FOR HOT DIP COATING OF METAL STRIP Filed Feb. 5. 1948 W. B Y a s max a w E w wwflm S 5 v 0 F 2 4 .YM

Patented Dec. 11, 1951 METHOD FOR no'r DIP COATING F METAL STRIP Carlos McNabb, Paul J. Whitley, and Forrest M. Morrow, Middletown, Ohio, assignors to Armco Steel Corporation, a corporation of Ohio Application February 3, 1948, Serial No. 5,958 6 Claims. (01. 117-64) This invention relates to exit rolls for hot dip coating of metal strip and to a method for the hot dip coating of metal strip.

In the coating of metal strip it is conventional to pass the strip through a bath of molten coating metal and to withdraw it from the bath through a pair of exit rolls the purpose of which is to control the thickness of the coating to be produced. Such rolls are usually uniformly grooved so as to permit a certain amount of coating metal to flow through them up above the bite of the rolls where it forms a meniscus of molten coating metal between the roll and the strip. The uniformly grooved rolls heretofore used have proved satisfactory at ordinary or conventional strip speeds. It has been found, however, that as strip speeds are increased, there is a tendency for the metal level to be drawn up slightly higher at the edges of the strip than the normal metal level of the bath while the level of the meniscus at the center of the strip i somewhat lowered because there is not sufficient time for the metal to flow up through the grooves to replace that beingwithdrawn by the strip. This results in the weight of coating along one edge portions of the strip being heavier than in the central portion. As the speed is increased, a point is reached where the variation in coating thickness becomes so great that the resulting strip not only has nonuniform corrosion resistance but becomes dimcult to handle. When such a strip is coiled, the edges of the coil will build up to larger diameters than the center which will stretch the edges until the strip when flattened out will have wavy edges. If such a strip is temper rolled following coating, the resulting strip or sheet will be reduced more at the edges and will not be flat.

The strip speed at which the problem above becomes of such serious proportions that it is in effect a limiting speed, has come to be known as the critical speed and it has been found that the critical speed is dependent primarily on the diameter of the exit rolls. We have found for example that with exit rolls which are l0-in. in diameter, the critical speed is about 80 ft. per minute. If it is attempted to coat metal strip at a speed above 80 ft. per minute with exit rolls 10-in. in diameter, the resulting strip is very non-uniformly coated and may be commercially unsalable because of the above mentioned heavy edge.

With the foregoing considerations in mind, it is an object of our invention to provide exit rolls of such construction that metal strip may be coated at speeds in excess of the critical speed and yet a substantially uniform weight of coating transversely of the strip may be obtained.

It is another object of our invention to provi'cle a method for the hot dip coating of metal strip at speeds above the critical speed.

These and other objects of our invention which we shall point out in more detail hereinafter or which will be apparent to those skilled in the art upon reading these specifications, we accomplish by that certain construction and arrangement of parts and by that series of method steps of which we shall now describe exemplary embodiments.

Reference is made to the drawings forming a part hereof and in which:

Figure 1 is a schematic, vertical cross-sectional view through a typical hot dip coating pot showing the location of exit rolls which form the subject matter of our invention.

Figure 2 is a fragmentary cross-sectional View taken on line 2-2 of Figure 1.

Figure 3 is an elevational view of one form of exit roll according to our invention.

Figure 4 is a view similar to Figure 3 of another form of exit roll according to our invention.

In Figure l, we have shown diagrammatically a conventional pot for molten coating metal at I!) having the usual roll I I under which the strip 12 passes. The exit rolls according to our invention are indicated at l3.

In Figure 2 we have shown the condition of the molten metal in the region between the rolls l3 in relation to the strip as it passes upward between the rolls. The general level of metal in the pot is indicated at It, and it will be observed that at the edge portions l5 of the strip, the level of the bath is raised as indicated at l6, while at the center of the strip the level of the bath is depressed as indicated at H. This condition is one which is encountered with ordinary or conventional exit rolls when the strip is moving in excess of the critical speed. It will be understood that Figure 2 is a somewhat exaggerated view to bring out more clearly the points involved.

Briefly, our method involves moving a strip through the bath of coating metal at a speed in excess of the critical speed and controlling or restricting the amount of coating metal withdrawn by the edge portions l6 of the strip in relation to the amount of metals withdrawn by central portions thereof, so as to counteract in effect the non-uniform level of the meniscus shown in Figure 2, and to produce a substantially uniform weight of coating on the strip l2 transversely thereof.

While this might be done in other ways, we prefer to do it by special grooving of the exit rolls so as to provide end portions of the rolls with a smaller cross-sectional groove area per unit of roll length than the cross-sectional groove area per unit of roll length of a central portion of the roll. The provision of a smaller crosssectional groove area per unit of roll length for the end portions of the roll can be achieved in steps or progressively, and it can be achieved by variation in the spacing of the grooves, the

the grooves are of the same width and the same depth, but the central grooves are closer together and succeeding grooves outward toward.

the ends of the roll are spaced progressively farther and farther apart. For example, a roll according to Figure 3 might have its central pair of grooves l-in. apart on centers, and the spaces between the adjacent grooves on either sides thereof might be le s-in and thus each succeeding groove outward toward the ends of the roll might be spaced from its adjacent groove by LEE-in. more than that groove was spaced from its preceding groove. Thus if'the roll were 55-in. long, the outermost grooves would be spaced 2-in. apart.

In Figure 4, we have shown another embodiment where the roll is indicated at I3?) and the grooves are indicated at 18b. Here the grooves are all of the same width and are uniformly spaced, but the central grooves are deepest and succeeding grooves to each side of the center become progressively more shallow.

While we have shown only two specific groove arrangements, it will be clear that others could be provided to accomplish the same purpose and to provide for a smaller cross-sectional groove area per unit of roll length at the end of the roll than at the center portion 'of the roll. Regardless of the manner in which the rolls are grooved, so long as it is in accord with the fundamental principles set forth herein, they counteract the non-uniform level of the meniscus discussed above in connection with Figure 2, and allow more coating metal to pass between them in the center of the strip than at the edges, and thus a uniform weight of coatin is produced transversely of the strip.

It will be clear that numerous modifications may be made in the specific arrangement of the grooves without departing from the spirit of our invention, and we thereforev do not intend to limit ourselves except as pointed out in the claims which follow.

Having now fully described our invention, what we claim as new and desire to secure by Letters Patent is:

l. The method of hot dip coating of metal strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls, the cross sectional groove area perunit of roll length of end portions of said rolls being less than the cross sectional groove area per unit of roll length of central portions thereof, at a speed above the critical speed, so that a substantially uniform weight of coatin is produced on said strip transversely thereof.

2. The method of hot dip coating of meta strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls, said rolls being characterized by a progressive decrease in cross sectional -groove area per unit of roll length in both directions from the roll centers. at a speed above the critical speed, so that a substantially uniform weight of coating is produced on said strip transversely thereof.

3. The method of hot dip coating of metal strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls, said grooves being separate and of equal cross sectional area, the spacing between adjacent grooves increasing progressively in both directions from the roll center, at a speed above the critical speed, so that a substantially uniform weight of coating is produced on said strip transversely thereof. 4. The method of hot dip coating of metal strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls, the two central grooves of each roll being on the order of one inch apart and the spacing between succeeding grooves outwardly from the roll centers increasing by about one-sixteenth of an inch per groove, at a speed above the critical speed, so that a substantially uniform weight of coating is produced on said strip transversely thereof.

5. The method of hot dip coating of metal strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls; said grooves being equally spaced longitudinally of said rolls and said grooves being of progressively increasing cross sectional area from the roll ends toward the roll centers, at a speed above the critical speed, so that a substantially uniform weight of coating is produced on said strip transversely thereof.

6. The method of hot dip coating of metal strip which includes the steps of running said strip through a bath of coating metal and through peripherally grooved exit rolls, said grooves being equally spaced longitudinally of the rolls andsaid grooves being of progressively increasing cross sectional area from the roll ends toward the roll centers by virtue of a progressive increase in groove depth, at a speed above the critical speed, so that a substantially uniform weight of coating is produced on said strip transversely thereof.

CARLOS MCNABB. PAUL J. WHITLEY. FORREST M. MORROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITEDSTATES PATENTS Number Name Date 1,980,961 Sommer Nov. 13, 1934 2,315,150 Bobrov Mar. 30, 1943 2,364,904 Keller Dec. 12, 1944 

2. THE METHOD OF HOT DIP COATING OF METAL STRIP WHICH INCLUDES THE STEPS OF RUNNING SAID STRIP THROUGH A BATH OF COATING METAL AND THROUGH PERIPHERALLY GROOVED EXIT ROLLS, SAID ROLLS BEING CHARACTERIZED BY A PROGRESSIVE DECREASE IN CROSS SECTIONAL GROOVE AREA PER UNIT OF ROLL LENGTH IN BOTH DIRECTIONS FROM THE ROLL CENTERS, AT A SPEED ABOVE THE CRITICAL SPEED, SO THAT A SUBSTANTIALLY UNIFORM WEIGHT OF COATING IS PRODUCED ON SAID STRIP TRANSVERSELY THEREOF. 